The present invention relates to a device, process and apparatus for stamping a plastic material using a die to form a shaped article. In particular, it relates to a process a for stamping a detergent bar.
By xe2x80x9cdetergent barxe2x80x9d is meant a tablet, cake or bar in which the level of surface active agent, which comprises soap, synthetic detergent active or a mixture thereof, is at least 20 wt. % based on the bar.
In the manufacture of detergent bars, a preformed composition comprising all components of the bar is typically extruded from a nozzle to form a continuous xe2x80x9crodxe2x80x9d which is cut into smaller pieces of predetermined length, commonly referred to as xe2x80x9cbillets.xe2x80x9d These xe2x80x9cbilletsxe2x80x9d are then fed to a stamper or, alternatively, are given an imprint on one or more surfaces using, for example, a die of the same dimensions as the bar surface. The bar surface is hit with force by, eg., a mallet or a die in the shape of a roller.
Stampers typically have a die formed in two halves each with a surface which contacts the billet during the stamping operation. These surfaces are adapted to close to a preset separation distance, thereby compressing the billet between the die halves to give the bar its final shape and appearance, and then separate. Excess composition is squeezed out from the die halves as they close. This is commonly referred to as xe2x80x9cflash.xe2x80x9d The flash is then separated from the soap bar by transferring the bar through holes in a xe2x80x9cdeflashing plate.xe2x80x9d
Conventional die stamping machines include xe2x80x9cpin diexe2x80x9d shaped machines in which a pair of opposing die members or die halves meet during a compaction step and a xe2x80x9cbox diexe2x80x9d machine in which a pair of opposing die members stamp a bar held within a through-opening in a box frame but do not meet during compaction, the peripheral face of the bar being restrained by the box frame.
The die halves are often each provided with a die or ejector insert. These are normally held closed within the die half by springs but can be pushed open by compressed air or mechanical means to assist in the release of the bar from the die. During closing of the die halves a vacuum can be applied to remove air trapped in the die cavity between the detergent bar and die surface and, in the case of rotary dies, this vacuum assists in retaining the bars in place during rotation.
Stamping of detergent bars using a die is carried out to give the bars a reproducible shape, smooth surface and/or to imprint a design such as a logo, trade mark or the like onto at least part of a surface of the bar.
However, as a result of die-blocking, i.e. amounts of residual detergent left on die halves which builds up during continued use of the dies, bars are often formed with visible imperfections on their surfaces or they may not release from the die surface.
Numerous solutions to these problems have been proposed. One solution involves chilling the die halves during the stamping operation. Chiling uses valuable resources, particularly energy sources.
Another solution is described in GB-A-746 769 which discloses a die set which includes a die box and a pair of companion die members made of plastic materials comprising polymers with a specified modulus of elasticity. A disadvantage of this system is that a die release agent is necessary to prevent detergent from adhering to and building up on the dies and marring the surfaces of subsequently pressed bars.
Luckow, U.S. Pat. No. 1,817,157 discloses a soap frame.
Pinto, U.S. Pat. No. 3,127,457 discloses a fluorocarbon film used as a release agent for molding polyurethane articles.
Krugg, U.S. Pat. No. 3,539,144 discloses a mold useful for producing polyurethane foams, which includes a structural mold shape and a coating of hot vulcanized silicon rubber disposed on the structural mold as well as a cold vulcanized silicone rubber deposited on the hot vulcanized silicon rubber.
Ladney, U.S. Pat. No. 3,889,919 discloses a plastic vent insert for a molding chamber for molding plastic parts. Molding cavity 26 includes a peripheral lip 28.
Cavanugh, U.S. Pat. No. 4,035,122 discloses a soap saving device for compressing remnants of soap bars.
Austin, U.S. Pat. No. 4,076,207 discloses a food mold comprising a liner and a rigid holder wherein the liner has a rim for securement to the holder.
Roussel, U.S. Pat. No. 4,809,945 discloses a mold cavity for use in molding articles such as soap bars.
Watanabe, U.S. Pat. No. 5,332,190 discloses an elastic molding die composed of an elastomer laminate film, the innermost layer of which does not contain fillers.
In U.S. Pat. No. 5,269,997 it is proposed to provide each of two dies of a soap mold with an elastomeric septum stretched across their surfaces. Such a system would be complex to use at the speed required for commercial manufacture and a thin covering would be prone to tearing and logo reproduction would be expected to be poor.
WO 96/00278 discloses a device for stamping a detergent bar comprising a die, the die having at least one bar stamping surface wherein the bar stamping surface is provided with an elastomeric coating, the total thickness of the elastomeric coating being less than 200 microns In a preferred embodiment, the elastomeric coating is the sole elastomeric material of the bar stamping surface.
Another solution is proposed in EP 276 971 and U.S. Pat. Nos. 4,793,959 and 4,822,273, which involve the use of two die members, each comprising a non-elastomeric and an elastomeric part. The elastomeric part, which contacts the soap bar during the stamping process, comprises an elastomer coating of at least 200 microns and having a modulus of elasticity within a specified range.
The manufacture of thick coated dies is a complex and expensive process. Therefore, it is highly desirable to maximize the working life of such coatings.
A difficulty which has been encountered with elastomeric coatings is that they tend to have short useful lives, on the order of a few days or more. The problems which this causes will be apparent.
It has been found that short life spans of elastomeric die coatings result from the high stress experienced by the edge of the die during the soap pressing operation. This results from the high soap stamping shear forces which tend to tear the elastomer coating away from the metal base of the dye. This problem tends to negate the advantages of use of the elastomeric coated dies, e.g. the ability to avoid the use of expensive cooled metal dies.
It has been discovered that the stress to which the elastomeric coatings are subjected can be reduced by use of an edge or flange, especially one which is itself similarly coated with elastomer. Preferably the edge or flange is made of a metal or other hard or rigid material. Use of the metal edge results in diminished tearing of the elastomer by lowering the stress to which it is subjected. The edge extends over the leading edge of the elastomer and thereby protects the elastomer from the applied stressed during stamping. This appreciably improves the useful life of the elastomer coated die and permits the advantages of the coated dies to be enjoyed without the disadvantage of an extremely short useful life.
Although it may be easier to make the edge of the same material as the die, this is not a necessity.
The edge or flange may be made of a multitude of materials, including plastics urethane and composites. In a preferred embodiment, the edge is provided by a xe2x80x9cTxe2x80x9d or umbrella shaped structure wherein the elastomer is protected beneath the top of the umbrella.
The invention is particularly useful for dies used to make detergent bars which have sidewalls which extend generally perpendicularly to the longitudinal axis of the bar, that is, xe2x80x9cbanded bars.xe2x80x9d Elastomeric dies for banded bars are believed to experience even higher stress than elastomeric dies for bars lacking a vertical sidewall or for which the vertical sidewall extends only a short percentage of the height of the bar.
It is contemplated that the edge or flange of the invention may also be used on a die which itself is made of an elastomeric material, rather than just coated with one. The edge will generally be made of a harder material than the die.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments and the accompanying drawings.